On April 4, 2014, Union Electric Company, doing business as Ameren
Missouri (Ameren Missouri), a subsidiary of Ameren Corporation (Ameren), issued and sold $350,000,000 principal amount of its 3.50% Senior Secured Notes due 2024 (the Notes), pursuant to a Registration Statement
on Form S-3 (File No. 333-182258-02), which became effective on June 21, 2012, and a Prospectus Supplement dated April 1, 2014 to a Prospectus dated June 21, 2012. Ameren Missouri received net offering proceeds of approximately
$347.6 million, before expenses, upon the closing of the transaction. Ameren Missouri intends to use the net offering proceeds to (i) repay at maturity $104 million aggregate principal amount of its 5.50% senior secured notes due May 15,
2014 and (ii) to repay a portion of its short-term debt (consisting of commercial paper issuances).

Ameren Missouri is filing this
Current Report on Form 8-K to report as exhibits certain documents in connection with the offering of the Notes.

Indenture dated as of August 15, 2002, from Ameren Missouri to The Bank of New York Mellon, as trustee, relating to the Notes (Current Report on Form 8-K, Exhibit 4.1, File No. 1-2967).

4.2

Company Order establishing the Notes.

4.3

Global Note.

4.4*

Indenture of Mortgage and Deed of Trust dated June 15, 1937, from Ameren Missouri to The Bank of New York Mellon, as trustee, as amended May 1, 1941, and Second Supplemental Indenture dated May 1, 1941 (Exhibit B-1, File No.
2-4940).

4.5

Supplemental Indenture, dated as of April 1, 2014, by and between Ameren Missouri and The Bank of New York Mellon, as trustee, relating to the First Mortgage Bonds, Senior Notes Series PP securing the Notes.

This combined
Form 8-K is being filed separately by Ameren Corporation and Union Electric Company (each a registrant). Information contained herein relating to any individual registrant has been filed by such
registrant on its own behalf. No registrant makes any representation as to information relating to any other registrant.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, each registrant has duly caused this report to be signed on its behalf by
the undersigned thereunto duly authorized. The signature for each undersigned company shall be deemed to relate only to matters having reference to such company or its subsidiaries.

Supplemental Indenture, dated as of April 1, 2014, by and between Ameren Missouri and The Bank of New York Mellon, as trustee, relating to the First Mortgage Bonds, Senior Notes Series PP securing the Notes.